Bicycles transmissions are used to transmit power applied to pedal cranks to the drive wheel. The pedal crank is typically located between the front and rear wheels, and may transmit power via a chain linked to the rear wheel and the pedal crank. There are typically two chain rings that a chain rides on. A first is by the pedal and a second on the rear wheel. Gear ratios between the pedal and the rear wheel gears are typically used to convert the speed and torque applied to the pedal to the wheel rotation. The gear by the pedal crank may also include a plurality of chain-rings and a front derailleur is used to move the chain across this plurality of chain-rings. It is also desirable to have smooth and continuous shifting between the plurality of chain-rings by the pedal crank. The rear wheel may further comprise a plurality of gears to create additional gear ratios for the bicycle. A gear shifter is typically used to move the chain across the plurality of gears on the wheel in order to set a desired gear ratio with the gear by the pedal. The desired gear ratio defines the two chain rings that the chain rides on.
In operating the bike, it is desirable that there be a smooth and continuous shifting between the plurality of gears on the wheel and the gear by the pedal in order to create the different gear ratios desired for a given ride. Sometimes, the smooth and continuous shifting may be hampered by gears that won't shift up or down in response to the gear shifter, by chain slipping or jumping or by shifting of the gears causing the chain to fall off.
In many of these applications, circular chain rings of various discrete radii are mounted on the pedal crank of the bicycle. Normally, circular chain-rings of various discrete radii are used on bicycles to achieve the effect of varying power transfer, but this method has the disadvantage of requiring the use of a gear derailleur, which rubs against the drive chain to change the chain's engagement on different chain-rings. Moreover, this change of chain-rings interrupts the power transfer, or at least makes the change abrupt, and can cause the chain to come off of the sprockets altogether. There is a need, therefore, to create a bicycle transmission which can change the radius of its gear sprockets in a continuous manner without creating friction on the drive chain while ensuring that the chain continues to be engaged throughout gear changes.
The major problem in implementing a transmission gear that can vary its radius is that as the radius increases, so does the circumference. Likewise, the distance between the teeth of the sprocket by which it is driven by the power train would also increase, yet the distance between the links of the drive chain which the teeth of the sprocket must engage does not, and cannot, increase if it is to effectively transfer power from the pedals and its sprocket. These difficulties may be overcome if the gear were broken into parts, so that the effect of continuously changeable gears was created by sliding the gear segments out from the center of the pedal crank's rotation.
Some prior attempts at solving these problems have sought to use gear segments to increase the radius of the sprocket, but these have involved complex mechanism to change the positions of these segments. U.S. Pat. No. 593,285 to Van Eyck in 1897 and U.S. Pat. No. 3,798,989 to Hunt in 1974 used gear segments actuated by springs and levers. U.S. Pat. No. 4,260,386 to Frohardt in 1981, U.S. Pat. No. 4,642,070 to Walker in 1987, and U.S. Pat. No. 4,772,250 to Kovar, et al. in 1988, attempted simultaneous displacement of gear segments through the actuation of a central cog and a series of springs, or an independent control disk. U.S. Pat. No. 5,772,546 to Warszewski in 1998 used the tension in the drive chain to vary the position of gear segments.
Other attempted solutions have used radially arranged bolts or screws to vary the radius of the element that engages a chain or belt in the transfer of power, but most have done so by expanding the whole circumference simultaneously. As mentioned above, simultaneous expansion of the circumference entails widening the space between the transmission elements which engage the drive chain or belt. U.S. Pat. No. 1,144,381 to Reimers in 1915, U.S. Pat. No. 4,167,124 to Zvetkov et al. in 1979, U.S. Pat. No. 4,696,662 to Gummeringer in 1987 and U.S. Pat. No. 4,740,190 to Pike in 1988 all attempted simultaneous expansion through radially arranged bolts actuated by a central beveled gear. U.S. Pat. Ser. No. 6,183,385 to Bakulich 2001 attempted the same sort of expansion either through a single electrical motor driving an independent control disk, or through a series of synchronized motors driving each bolt.
U.S. Pat. Ser. No. 5,476,422 to Schendel in 1995 attempted to effect the displacement of chain engaging elements along radially arranged bolts through the successive rotation of wheels located at the periphery of his device which drove the bolts. U.S. Pat. Ser. No. 5,984,814 to Davenport in 1999 employed movement of chain engaging sectors along radially arranged arms relying on springs to bias their motion outward, and braking a spooling element to wind flexible elements to draw the sectors inward. U.S. Pat. No. 4,634,406 to Hufschmid in 1987 used pivotal prongs positioned next to a disc to vary the radial position of the several sprocket segments sliding within radially arranged slots, the segments being locked in the desired position by a spring biased pin. U.S. Pat. No. 4,516,960 to Rathert, also in 1987 used a fork-shaped adjusting member to vary the radial position of chain engaging drivers which lock in the desired position by means of force applied to wedge-shaped elements from the tension on the driving chain.
U.S. Pat. No. 6,332,852 to Allard in 2001 employed a spring-biased locking pin which pressed against a sloped plate to release the adjusting element to which was attached a chain-engaging gear segment to allow for the change its radial position; the locking pin then was pushed along an angled face of the sloped plate to actually change its radial position. The overall mechanism consisted of many more parts than the present disclosure, and friction produced by pressing the locking pin against the sloped plate with each rotation of the device makes it impractical. PCT Publ. WO2013013270 to Kaiser and Braur in 2013 disclosed disengagement of locking pins and adjustment to new radial positions by means of magnets, but this system is both unreliable and excessively heavy, as well as being more complicated than the present disclosure. German patent application DE 102010019045 A1 to Hettlage in 2011 used belt engaging elements sliding within radially arranged grooves, but the mechanism by which they were locked into position or released to allow adjustment consisted of at least six parts, and so is considerably more complicated than the present disclosure.
What is needed is a transmission which will smoothly and reliably transition from a lower mechanical advantage at a large radius to a higher mechanical advantage at a lower radius, or vice-versa, without complicated gearing mechanisms. The transmission should be easy to install on an existing mechanical device, such as a bicycle, without complicated adjustments or adaptations.